The present invention relates to a rolling bearing suitable to automobiles and industrial machines such as construction machines and iron and steel making machines and, particularly, suitable for the use in environments of severe lubrication conditions (for example, at high temperature).
The present invention also relates to a rolling bearing suitable, for example, to {circle around (1)} rotational supporting use in information equipment such as hard disk drives (HDD), video tape recorders (VTR) and digital audio tape recorders (DAT), {circle around (2)} supporting use for swinging portions of swing arms as constituent components such as of HDD and {circle around (3)} rotational supporting use for those equipments requiring quietness such as motors for blowers, motors for cleaners and vehicle turbo chargers.
Furthermore, this invention relates to a rolling bearing suitable to use undergoing fitting stress to an inner ring relative to a shaft and used at high temperature and lubrication with intrusion of obstacles.
Heretofore, high carbon chromium steels such as SUJ 2 have been mainly used as steel materials for rolling bearings but high speed steel materials such as AISI-M50 have been used in such uses as requiring high performance at high temperature (for example, bearings for use in aircraft jet engines). However, since the materials contain many alloy elements and require complicate heat treatment steps, the cost of the obtained bearings is expensive.
Japanese Published Unexamined Patent Application Hei 3-253542 discloses a steel containing Si and Mo having high softening resistance at high temperature more than SUJ 2 and containing an appropriate amount of Cr. This publication discloses that the fatigue life characteristic of rolling bearings at high temperature can be improved while keeping the manufacturing cost lower by the use of such steels.
Further, Japanese Published Examined Patent Application Hei 6-33441 describes bearing rings formed by using a steel containing, on the weight ratio, 0.95 to 1.10% of carbon, 1 to 2% of silicon or aluminum, 1.15% or less of manganese and 0.90 to 1.60% of chromium, with an oxygen content of 13 ppm or less, with the amount of residual austenite being restricted to 8% or less by conducting tempering at a temperature of 230xc2x0 C. to 300xc2x0 C. after hardening and with a hardness of HRC60 or more. Further, this invention intends to provide a rolling bearing ring having high dimensional stability and extended rolling life even in use at high temperature.
Further, Japanese Published Unexamined Patent Application Hei 10-68419 describes a rolling bearing in which carbides and/or carbonitrides with the maximum grain size of 5 xcexcm or less are precipitated at the surface layer of bearing rings and/or rolling elements, and the surface hardness of the bearing ring and/or rolling element is Hv 600 or more and 700 or less at 300xc2x0 C. According to this rolling bearing, while wear resistance at high temperature can be improved, manufacturing cost is increased since a carburization treatment or carbonitriding treatment is required during manufacture.
However, the rolling bearings disclosed in each of the publications still have a room for the improvement in view of the wear resistant at high temperature or manufacturing cost.
On the other hand, rolling bearings used for rotational support in those equipments requiring quietness as in {circle around (1)}-{circle around (3)} described above are required to have satisfactory acoustic characteristics at low torque (less noise). Therefore, bearing components such as inner rings, outer rings and rolling elements are finished at high dimensional accuracy. Further, the inner rings, outer rings and the rolling elements are formed of high carbon chromium bearing steels such as SUJ 2 or martensitic stainless steels such as SUS440C and then manufactured by applying hardening tempering and the hardness of the raceway surface is defined as HRC 58 to 64.
In recent years, since information equipments have been reduced in size and often adapted for portable use, they have more worry of undergoing impact shocks upon dropping or exposure to vibrations. Correspondingly, a worry of damages has also been increased for rolling bearings in the equipments. In small-sized ball bearings used for portable information equipments, since the contact ellipsis formed at the contact surface between the bearing ring and the rolling element is small, when impact load is applied, the contact portion suffers from permanent deformation to sometimes cause indentation at the raceway surface even when this is a relatively small impact load. As a result, there may be a worry for the deterioration of acoustic characteristics or occurrence of unevenness in the rotational torque.
Prior art for overcoming the problems can include, techniques as described in Japanese Published Unexamined Patent Application Hei 7-103241 and Japanese Published Unexamined Patent Application Hei 8-312651.
Japanese Published Unexamined Patent Application Hei 7-103241 describes that the amount of the residual austenite in the steel forming the raceway surface is decreased as low as 6% by volume or less to improve the indentation resistance of the raceway surface thereby avoiding permanent deformation of the raceway surface when impact load is applied to the rolling bearing. For example, after forming a bearing ring with SUJ 2, it is hardened at a hardening temperature of standard heat treatment (820 to 860xc2x0 C.) and then subjected to sub zero treatment, or tempered at a relatively high temperature of 220 to 240xc2x0 C. thereby decreasing the amount of the residual austenite as less as possible while maintaining required hardness for the raceway surface.
Japanese Published Unexamined Patent Application Hei 8-312651 discloses that a bearing ring is formed with usual bearing steel (high carbon chromium bearing steel such as of case hardened steel or SUJ 1-3) and then applied with a carbonitriding hardening treatment and tempering at a temperature of 350xc2x0 C. or higher thereby reducing the amount of the residual austenite of steels forming the raceway surface to 0%, for improving the indentation resistance at the raceway surface.
It also discloses that the bearing ring is formed of a steel prepared by adding an element providing a tempering resistance and then applied with quench hardening and tempering at a temperature of 350xc2x0 C. or higher to reduce the amount of the residual austenite of the steel forming the raceway surface to 0%. Further, it discloses that no indentation by contact with the raceway surface is formed to the rolling element, by making the rolling element with ceramic material.
However, in the prior art described above, there is still a room for the improvement in view of the acoustic characteristics when impact load is applied.
On the other hand, self alignment roller bearings used, for example, in paper making machine are sometimes used under high fitting stress in excess of 100 MPa (average stress along the cross section of an inner ring for the tension applied in the circumferential direction of the inner ring) in order to prevent creeping caused between a shaft and a bearing inner ring. In such a case, an inner ring having an inner diametrical portion fabricated into a tapered shape is press fit into a tapered shaft so as to facilitate application of fitting stress. As the inner ring of such a shape, those prepared by applying quenching and tempering treatment to fully cured steels such as high carbon chromium bearing steels (C: about 1 wt %, Cr: about 1.5 wt % content) are generally used.
Then, when the inner ring comprising the fully hardened steel is used under fitting stress in excess of 100 MPa, the inner ring may sometimes be cracked in the axial direction with non-metal inclusions present near the raceway surface as initiation points depending on the combination of the fitting stress and the rolling stress.
For preventing such cracking fracture of the inner ring, there is a general knowledge that improvement of the compressive residual stress on the raceway surface or the fracture toughness of the material per se is effective. Based on this knowledge, the compressive residual stress at the raceway surface has been increased by applying an austemper treatment to fully hardened steels, or by using carburized steels.
However, among the prior art described above, the method of applying the austempering to the fully hardened steel can not prevent the crack fracture of the inner ring in a case of use under a high fitting stress as exceeding 130 MPa since the compressive residual stress capable of providing to the raceway surface by the austempering is about xe2x88x92100 MPa.
The method of using the carburized steel is effective also for preventing cracking fracture of the inner ring used under a fitting stress in excess of 130 MPa since a compressive residual stress of about xe2x88x92200 MPa can be provided to the raceway surface by controlling the conditions for carburization, hardening and tempering. However, in a case of applying the carburization to those steel materials with low carbon content, for example, of about 0.20% by weight, it involves a drawback that the time for carburization treatment is longer. Since the carburizing time is in proportion with the square of the carburizing depth, this results in a problem of worsening the productivity and increasing the cost, particularly, in medium to large size bearings requiring deep carburized layer.
In order to overcome this problem, Japanese Published Unexamined Patent Application Hei 6-307457 discloses a rolling bearing in which an inner ring is formed by carburizing or carbonitriding an alloy steel with a carbon content of 0.3 to 0.7% by weight, in which the carbon content at the surface layer of the inner ring on the side of the raceway surface (C1) is 1.3% by weight or less and the difference of the carbon content (C1) at the surface layer and the carbon content (C2) in the core portion (xcex94C=C1-C2) is 0.4% by weight or more.
Also for such a rolling bearing used under application of fitting stress to the inner ring, there is a demand for improving the life when it is used under high temperature and lubrication with intrusion of obstacles.
A first subject of the present invention is to provide a rolling bearing having high wear resistance under high temperature and with reduced manufacturing cost.
A second subject of the present invention is to provide a rolling bearing of excellent acoustic characteristics when undergoing impact loads, suitable for use in small sized information equipments for portable use.
A third object of the present invention is to provide a rolling element having an inner ring capable of enduring the use at high fitting stress exceeding 130 MPa, having a longer life under high temperature and lubrication with intrusion of obstacles and with less manufacturing cost.
(First Rolling Bearing)
In order to solve the first subject described above, the present invention provides a rolling bearing in which at least one of an inner ring (a shaft in a case where an inner ring raceway surface is formed to the shaft), an outer ring and an rolling element is formed of a steel material containing, as alloy ingredients, 0.8% by weight or more and 1.2% by weight or less of C, 0.5% by weight or more and 2.5% by weight or less of Si, 0.7% by weight or more and 1.5% by weight or less of Cr, 0.8% by weight or more and 2.0% by weight or less of Mo and 0.3% by weight or more and 1.2% by weight or less of Mn, with a ratio of Mo to Cr (Mo/Cr: weight ratio) of 1.1 or more into a predetermined shape, then applied with hardening, and then applied with tempering at a temperature of 240xc2x0 C. or higher and 350xc2x0 C. or lower to make the Vickers hardness (HV) at the raceway surface and/or rolling surface to 720 or more. The rolling bearing is referred to as a first rolling bearing.
According to the first rolling bearing, since at least one of the bearing members (inner ring, outer ring and rolling element) is formed of the steel material described above, wear resistance at high temperature can be increased without carburization treatment or carbonitriding treatment. Particularly, when the ratio of Mo and Cr contained in the steel material used is defined as Mo/Crxe2x89xa71.1, the formulation balance between Mo carbides and Cr carbides is improved to dispersingly precipitate fine carbides (M23C6 type), so that satisfactory wear resistance can be obtained.
Further, when the content for each of the ingredients in the steel material used is defined within the predetermined range and the tempering temperature is set to 240xc2x0 C. or higher and 350xc2x0 C. lower to make the Vickers hardness (HV) at the raceway surface and/or rolling surface to 720 or more, hardness and wear resistance capable of enduring high temperature use can be obtained.
Further, for suppressing the dimensional change upon long time use at high temperature, the amount of the residual austenite after tempering is preferably defined as 2.0% by volume or less. The tempering temperature has to be higher in order to decrease the amount of the residual austenite but hardness tends to be lowered as the tempering temperature goes higher. Accordingly, the hardness and the dimensional stability are compatibilized by defining the tempering temperature to 240xc2x0 C. or higher and 350xc2x0 C. or lower.
Critical meanings for the range of the content of each of the ingredients is to be described below.
(C: 0.8% by weight or more and 1.2% by weight or less)
C (carbon) is an element for providing steels with hardness. For ensuring a sufficient hardness for inner and outer rings and rolling elements of a bearing after the structure is made martensitic by hardening and after tempering at a temperature of 240xc2x0 C. or higher and 350xc2x0 C. or lower, the C content has to be 0.8% by weight or more.
Further, if the C content is excessively high, macro carbides tend to be formed. Since the macro carbides possibly form initiation points for flaking, the rolling fatigue life of the rolling bearing is lowered if the C content is excessively high. In order not to form macro carbides, the C content is defined as 1.2% by weight or less.
(Si: 0.5% by Weight or More and 2.5% by Weight or Less)
Si (silicon) is an element acting as a deoxidizer upon steel making and providing steels with anti-temperability. Such effects can not be obtained substantially if the Si content is less than 0.5% by weight.
Further, if the Si content is excessively high, cutting property or forgeability is deteriorated. Accordingly, the upper limit of the Si content is defined as 2.5% by weight (preferably, 2.0% by weight).
(Cr: 0.7% by Weight or More and 1.5% by Weight or Less)
Cr (chromium) is an element having an effect of improving the hardenability and providing the steel with a anti-temperability and it is also an element of forming chromium carbides. If the Cr content is less than 0.7% by weight, such effect can not substantially be obtained.
On the other hand, if the Cr content is excessively high, macro carbides tend to be formed, which cause lowering of the rolling contact fatigue life of rolling bearings. Further, when the Cr content is higher, the material cost is increased since an expensive Mo has to be added in a great amount so as to attain: Mo/Crxe2x89xa71.1. Further, if the Cr content exceeds 1.5% by weight, the effect of improving the wear resistance obtained by the relation: Mo/Crxe2x89xa71.1 is saturated. Accordingly, the Cr content is defined as 1.5% by weight or less.
(Mo: 0.8% by Weight or More and 2.0% by Weight or Less)
Mo (molybdenum) is an element of providing the steel with anti-temperability and it is also an element of forming carbides. If the Mo content is less than 0.8% by weight, such effects can not be obtained substantially.
On the other hand, if the Mo content is excessively high, the workability is lowered. Further, if the Mo content exceeds 2.0% by weight, the effect of improving the wear resistance obtained by making: Mo/Crxe2x89xa71.1 is saturated to merely result in increased material cost. Accordingly, the Mo content is defined as 2.0% by weight or less.
(Mn: 0.3% by Weight or More and 1.2% by Weight or Less)
Mn (manganese) is an element having an effect of improving the hardenability, which is an element essential to bearing steels. If the Mn content is less than 0.3% by weight, no sufficient effect can be obtained.
On the other hand, if the Mn content exceeds 1.20% by weight, the workability is lowered and inclusions possibly causing lowering of the bearing life tend to be formed. Accordingly, the Mn content is defined as 1.2% by weight or less.
(Second Rolling Bearing)
For solving the second subject described above, the present invention provides a rolling bearing having an outer ring, an inner ring or a shaft in a case where an inner ring raceway surface is formed to a shaft in which at least one of the inner ring (or shaft) and the outer ring is formed of a steel material containing, as the alloy ingredients, 0.8% by weight or more and 1.2% by weight or less of C, 0.5% by weight or more and 2.5% by weight or less of Si, 0.7% by weight or more and 1.5% by weight or less of Cr, 0.8% by weight or more and 2.0% by weight or less of Mo and 0.3% by weight or more and 1.2% by weight or less of Mn into a predetermined shape, then applied with hardening and tempering, with a ratio of Mo to Cr (Mo/Co; weight ratio) at a position from the surface of a raceway surface to a core portion by a size corresponding to 2% of a diameter of a rolling element (position for 2% Da depth) is 1.1 or more, a Vickers hardness (HV) at the position is 720 or more and the amount of the residual austenite at the position is 1.0% by volume or less. The rolling bearing is referred to as a second rolling bearing.
xe2x80x9cElastic Coefficient of Metal Materialxe2x80x9d page 11, book published from The Japan Society of Mechanical Engineers (October 1980) describes that xe2x80x9cReferring to the elastic coefficient of a solid solubilized alloy, when solute atoms are interstitial atoms the elastic coefficient is always lowered since crystal lattices are disturbed greatlyxe2x80x9d. That is, even when the hardness of the raceway surface is identical, the elastic coefficient of the bearing ring is lowered to lower the contact pressure between the bearing and the rolling element as the crystal lattice in the steel constituting the bearing ring is disturbed more, the indentation resistance is improved.
On the other hand, According to xe2x80x9cMetal Data Bookxe2x80x9d published from The Japan Institute of Metals, page 8 (July 1974), the atomic radius of iron (Fe) is 1.24 xc3x85, the atomic radius of chromium (Cr) is 1.25 xc3x85 and the atomic radius of molybdenum (Mo) is 1.36 xc3x85. Further, for both chromium and molybdenum, Cr and Mo atoms are substituted for Fe atoms when they are solid solubilized in iron. In this case, when the Cr atom having substantially the same atomic radius as iron is replaced for the Fe atom, no significant distortion is caused to the crystal lattice in the steel. When Mo atom is substituted for the Fe atom, since there is a difference of the atomic radius between both of them, large distortion is caused to the crystal lattice in the steel if hardening is caused by martensitic transformation while Mo is solid solubilized as it is.
However, if only Mo is added in a great amount, it is bonded with C to form Mo carbides and the amount of Mo solid solubilized into the matrix is not sometimes increased. Further, since Mo is an expensive material compared with Cr, it is desirable that the addition amount is kept as less as possible.
The present inventors have found that Mo is efficiently solid-solubilized in the matrix by making the Mo to Cr ratio (Mo/Cr: weight ratio) in the steel constituting a bearing ring to 1.1 or more thereby enabling to make the distortion of the crystal lattice greater in the steel constituting the bearing ring.
The indentation resistance is particularly improved by increasing the elasticity forming performance at a position in the direction of the depth of the raceway surface where a maximum shearing stress is applied. The position in the direction of the depth of the raceway surface where the maximum sharing stress is applied varies depending on the factors in the bearing design and working conditions, and it is typically xe2x80x9cposition from the raceway surface to a core portion corresponding to 2% of a diameter of a rolling element (position for 2% Da depth)xe2x80x9d.
Therefore, according to the second rolling bearing of present invention, particularly preferred indentation resistance can be obtained by not only defining the Vickers hardness (HV) as 720 or more (preferably, 750 or more) at the position for 2% Da depth and reducing the amount of the residual austenite at that position to 1.0% by volume or less but also making the ratio of Mo to Cr (Mo/Cr: weight ratio) at that position to 1.1 or more.
Further, vanadium (V) having an atomic radius of 1.31 xc3x85 and tungsten (W) having an atomic radius of 1.37 xc3x85 can also cause great distortion to the crystal lattice in the steel like that molybdenum (Mo) since they have a difference in the atomic radius relative to iron. Accordingly, a steel material further containing V and/or W in addition to each of the ingredients described above may also be used. In this case, the ratio for the total content of Mo, V and W to Cr ((Mo+V+W)/Cr: weight ratio) at the position for 2% Da depth is defined as 1.1 or more.
The upper limit for the ratio of Mo to Cr at the position for 2% Da depth is a value (2.86) calculate based on the lower limit for the Cr content (0.7% by weight) and the upper limit for the Mo content (2.0% by weight) in the steel material to be used.
In the second rolling bearing according to the present invention, at least one of the inner ring (or shaft) and the outer ring is formed of a steel material containing, as the alloy ingredient, 0.8% by weight or more and 1.2% by weight or less of C, 0.5% by weight or more and 2.5% by weight or less of Si, 0.7% by weight or more and 1.5% by weight or less of Cr, 0.8% by weight or more and 2.0% by weight or less of Mo and 0.3% by weight or more and 1.2% by weight or less of Mn.
The critical meaning for the range of each of the contents of the ingredients in the second rolling bearing is basically identical with the case of the first rolling bearing described above. However, the description for the Mo content that xe2x80x9cthe effect of improving the wear resistance obtained by defining as: Mo/Crxe2x89xa71.1xe2x80x9d should be changed in the second rolling bearing as xe2x80x9cthe effect of improving the impact resistance obtained is saturated by defining as: Mo/Crxe2x89xa71.1xe2x80x9d.
In the second rolling bearing according to the present invention, it is preferred to apply a sub-zero treatment after hardening to induce transformation of residual austenite into martensite, thereby reducing the amount of the residual austenite and then apply tempering. When the residual austenite is decomposed as much as possible before tempering by applying the sub-zero treatment before tempering, distortion of the lattice in the martensitic structure after hardening is increased to promote the effect of reducing the elastic coefficient described above.
In the second rolling bearing according to the present invention, it may suffice that at least the inner ring (shaft in a case where the inner ring raceway surface is formed to the shaft) in the inner ring and the outer ring has the foregoing constitution but it is preferred that both of the inner ring and the outer ring have the constitution described above.
In the second rolling bearing according to the present invention, the rolling element has no particular restriction and it may be made of SUJ 2 used so far, made of ceramics or made of stainless steels. When the rolling element is made of ceramics, fretting resistance is improved outstandingly compared with the case where the rolling element is made of metal. However, when the bearing ring is an existent product, since a large contact pressure is caused between the raceway surface and the ceramic rolling element, the indentation resistance at the raceway surface is lowered.
On the contrary, in the second rolling bearing according to the present invention, since the bearing ring is constituted as described above, the indentation resistance at the raceway surface can be kept high even when the rolling element is made of ceramics. Accordingly, in the rolling bearing of the present invention, the fretting resistance can be improved while keeping the indentation resistance at the raceway surface high by using a rolling element made of ceramics.
As the rolling element constituting the second rolling bearing according to the present invention, a rolling element formed of a stainless steel containing 8% by weight or more (preferably 12% by weight or more) of Cr, in which a nitride layer is formed on the surface by a nitriding treatment and a core portion has a total content of nitrogen and carbon of 0.45% by weight or more is preferred. Softening of the core portion by high temperature tempering after the nitriding treatment can be prevented by making the total content for nitrogen and carbon in the core portion to 0.45% by weight or more.
Further, when nitride forming elements such as Mo, V, W, Nb, Al and Si are further added to the stainless steel, since fine nitrides thereof are formed, durability of the rolling element is further improved, so that such elements are preferably added in an appropriate amount in view of the cost.
Further, as a combination of the rolling element and the bearing ring (an inner ring or a shaft having the inner ring raceway surface and an outer ring), it is preferred that the difference between the surface hardness of the rolling element and the hardness of the bearing ring at a position for 2% Da depth is HV 300 or more and, more preferably, HV 500 or more. This can provide a higher effect for improving the impact resistance (indentation resistance).
(Third Rolling Bearing)
For solving the third subject described above, the present invention provides a rolling bearing in which an inner ring has the following constitution. The inner ring is formed of a steel material containing as, the alloy ingredient, 0.3% by weight or more and 0.7% by weight or less of C and 0.7% by weight or more and 1.5% by weight of less of Cr into a predetermined shape, applied with a carbonitriding treatment and hardening and tempering, in which a Vickers hardness (HV) at the raceway surfaces 700 is or more and an absolute value for the compressive residual stress at the raceway surface is 160 MPa or more. The rolling bearing is referred to as a third rolling bearing.
A carburizing treatment may be applied instead of the carbonitriding treatment so long as xe2x80x9cVickers hardness (HV) at the raceway surface of 700 or more, absolute value of a compressive residual stress at the raceway surface of 160 MPa or morexe2x80x9d is obtained.
According to the third rolling bearing, since the absolute value for the compressive residual stress at the raceway surface of the inner ring is 160 MPa or more, cracking fracture of the inner ring is prevented even in a case of use under application of high fitting stress of 130 MPa or more to the inner ring. If the carbon content in the steel material used exceeds 0.7% by weight, the absolute value for the compressive residual stress at the raceway surface is less than 160 MPa. If the carbon content is less than 0.3% by weight, it takes much time for the carbonitriding treatment.
A carbonitriding time required for applying a predetermined amount of carbonitridation to an identical depth was examined while changing the carbon content in the alloy steel used (1.0% by weight of C at the surface and 0.75% by weight of C at 1 mm depth). The result is shown by a graph in FIG. 7. As can be seen from the graph, if the carbon content is 0.3% by weight or lower, the time required for the carbonitriding treatment is extremely long. In a case of the carbon content of 0.2% by weight, it takes about twice time compared with the case of 0.3% by weight.
Further, if the chromium content in the steel material used is less than 0.7% by weight, the wear resistance is insufficient and, if it exceeds 1.5% by weight, macro carbides tend to be formed which may cause lowering of the rolling fatigue life of the rolling bearing.
Further, since the Vickers hardness (HV) at the raceway surface of the inner ring is 700 or more, the high temperature fatigue life and the wear resistance are satisfactory. It is preferred that the Vickers hardness (HV) at the raceway surface of the inner ring is 720 or more.
The carbon content at the surface layer of the inner ring on the side of the raceway surface (surface carbon concentration on the inner ring raceway surface) is preferably 0.8% by weight or more and 1.3% by weight or less.
If the surface carbon concentration at the inner ring raceway surface is less than 0.8% by weight, the rolling fatigue life at high temperature is insufficient. If the surface carbon concentration at the inner ring raceway surface exceeds 1.3% by weight, macro carbides are formed in the raceway surface to lower the rolling life.
In the third rolling bearing, it is preferred that the outer ring is formed of a steel material containing, as the alloy ingredient, 0.8% by weight or more and 1.2% by weight or less of C, 0.5% by weight or more and 2.5% by weight or less of Si, 0.7% by weight or more and 1.5% by weight or less of Cr, 0.8% by weight or more and 2.0% by weight or less of Mo and 0.3% by weight or more and 1.2% by weight or less of Mn, with a ratio of Mo to Cr (Mo/Cr: weight ratio) of 1.1 or more into a predetermined shape, then applied with hardening and then applied with tempering at a temperature of 240xc2x0 C. or higher and 350xc2x0 C. or lower to make the Vickers hardness (HV) at the raceway surface and/or rolling surface to 720 or more. The rolling bearing is referred to as a fourth rolling bearing.
According to the fourth rolling bearing, since the outer ring is formed of the steel material described above, wear resistance at high temperature can be improved with no carburization treatment or carbonitriding treatment. Particularly, formulation balance between Mo carbides and Cr carbides is improved by defining the ratio of Mo to Cr contained in the steel material used as Mo/Crxe2x89xa71.1, and fine carbides (M23C6 type) are dispersingly precipitated to obtain satisfactory wear resistance.
In this fourth rolling bearing, the critical meaning for the range of the content of each of the ingredients in the steel material constituting the outer ring is identical with that in the first rolling bearing described above.
Further, hardness capable of enduring high temperature use and wear resistance can be obtained by defining the content for each of the ingredients in the steel material used to the predetermined range, and controlling the tempering temperature to 240xc2x0 C. or higher and 350xc2x0 C. or lower and making the Vickers hardness (HV) at the raceway surface to 720 or more.
Therefor, since the fourth rolling bearing comprises an inner ring capable of enduring use under high fitting stress exceeding 130 MPa and an outer ring having a high wear resistance at a high temperature, it is suitable as a rolling bearing to be used under application of a fitting stress to the inner ring relative to the shaft and for use under high temperature and lubrication with intrusion of obstacles. Furthermore, since the outer ring is not applied with carburization treatment or carbonitride treatment, manufacturing cost is reduced.
Further, for suppressing the dimensional change upon long time use at a high temperature, it is preferred that the amount of the residual austenite after tempering is 2.0% by volume or less for the outer ring of the fourth rolling bearing. It is necessary to make the tempering temperature higher in order to decrease the amount of the residual austenite, but the hardness tends to lower as the tempering temperature is higher. Accordingly, the hardness and the dimensional stability are compatibilized by defining the tempering temperature to 240xc2x0 C. or higher and 350xc2x0 C. or lower.
Also in the fourth rolling bearing, it is preferred to restrict the amount of the residual austenite after tempering to 2.0% by volume or less for suppressing the dimensional change upon long time use at high temperature.
For solving the third subject, this invention also provides a rolling bearing having an inner ring and an outer ring of the following constitution (fifth rolling bearing). The an inner ring is formed of a steel material containing, as the alloy ingredients, 0.3% by weight or more and 0.7% by weight or less of C, 0.7% by weight or more and 1.5% by weight or less of Cr and 0.8% by weight or more and 2.0% by weight or less of Mo, with a ratio of Mo to Cr (Mo/Cr: weight ratio) of 1.1 or more into a predetermined shape, then applied with a carbonitriding treatment and hardening and tempering to make the Vickers hardness (HV) at the raceway surface of 720 or more, and the absolute value for the compressive residual stress on the raceway surface is 160 MPa or more.
The outer ring of the fifth rolling bearing is identical with outer ring of the fourth rolling bearing.
According to the fifth rolling bearing, since the ratio of Mo to Cr contained is defined as; Mo/Crxe2x89xa71.1 not only in the steel material for the outer ring but also for the steel material for the inner ring, the wear resistance of both of the inner ring and the outer ring is improved. Accordingly, it has a longer life under high temperature and lubrication with intrusion of obstacles than the fourth rolling bearing.
Also for the inner ring and the outer ring of the fifth rolling bearing, it is preferred for suppressing dimensional change upon long time use under high temperature to define the amount of the residual austenite after tempering to 2.0% by volume or less.
The present invention also provides a rolling bearing as defined in the third rolling bearing in which the outer ring is formed of a steel material containing, as the alloy ingredient, 0.3% by weight or more and 0.7% by weight or less of C and 0.7% by weight or more and 1.5% by weight or less of Cr into a predetermined shape and then applied with a carbonitriding treatment and hardening and tempering in which the Vickers hardness (HV) at the raceway surface is 700 or more and an absolute value for the compressive residual stress at the raceway surface is 160 MPa or more.
Also for the inner ring and the outer ring in this rolling bearing, it is preferred for suppressing dimensional change upon long time use at high temperature to define the amount of the residual austenite after tempering to 2.0% by volume or less.